5-Arylhydantoins have been classically known to be synthesized in accordance with Bucherer-Berg method by which the corresponding arylaldehyde is reacted with ammonium carbonate and sodium cyanide (see "J. Prakt. Chem.", page 291, 140[1934]).
Many other synthesis methods have been known such as a method which comprises the reaction of glyoxylic acid, urea and an aryl compound under acidic conditions (see JP-A-54-106740 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-B-55-22474 (the term "JP-B" as used herein means an "examined Japanese patent publication")), a method which comprises the reaction of 5-hydroxyhydaintoin with an aryl compound (see JP-A-54-138560), a method which comprises the reaction of allantoin with phenol (see JP-B-55-16582), a method which comprises the reaction of the reduction product of parabanic acid with phenol (see JP-A-3-206080) and a method which comprises the reaction of 4-hydroxyphenylketoacetal with phenol (see JP-A-4-364150).
Bucherer-Berg method requires the use of sodium cyanide, which is very dangerous. This method is also disadvantageous in that the resulting crude hydantoin contains a large amount of products of the secondary reaction occurring with the oxidation of phenol ring under alkaline conditions and thus is liable to coloration.
The synthesis methods which comprise the use of starting materials such as glyoxylic acid, 5-hydroxy-hydantoin, allantoin, parabanic acid and 4-hydroxyphenylketo-acetal leave something to be desired because these starting materials are expensive.